ALMA and VLA measurements of frequency-dependent time lags in Sagittarius A*: evidence for a relativistic outflow

¹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
e-mail: c.brinkerink@astro.ru.nl
² Department of Astronomy and Radio Astronomy Lab, University of California, Berkeley, CA, USA
³ Joint ALMA Observatory, ESO, Santiago, Chile
⁴ Academia Sinica Institute of Astronomy and Astrophysics, 645 N. A’ohoku Pl., Hilo, HI 96720, USA
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ Astronomy Department, University of Illinois, 1002 West Green Street, Urbana, IL 61801, USA
⁷ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁸ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁹ Department of Physics, Astrophysics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
¹⁰ School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
¹¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 8 August 2014
Accepted: 3 February 2015

Abstract

Context. Radio and mm-wavelength observations of Sagittarius A* (Sgr A*), the radio source associated with the supermassive black hole at the center of our Galaxy, show that it behaves as a partially self-absorbed synchrotron-emitting source. The measured size of Sgr A* shows that the mm-wavelength emission comes from a small region and consists of the inner accretion flow and a possible collimated outflow. Existing observations of Sgr A* have revealed a time lag between light curves at 43 GHz and 22 GHz, which is consistent with a rapidly expanding plasma flow and supports the presence of a collimated outflow from the environment of an accreting black hole.

Aims. Here we wish to measure simultaneous frequency-dependent time lags in the light curves of Sgr A* across a broad frequency range to constrain direction and speed of the radio-emitting plasma in the vicinity of the black hole.

Methods. Light curves of Sgr A* were taken in May 2012 using ALMA at 100 GHz using the VLA at 48, 39, 37, 27, 25.5, and 19 GHz. As a result of elevation limits and the longitude difference between the stations, the usable overlap in the light curves is approximately four hours. Although Sgr A* was in a relatively quiet phase, the high sensitivity of ALMA and the VLA allowed us to detect and fit maxima of an observed minor flare where flux density varied by ~10%.

Results. The fitted times of flux density maxima at frequencies from 100 GHz to 19 GHz, as well as a cross-correlation analysis, reveal a simple frequency-dependent time lag relation where maxima at higher frequencies lead those at lower frequencies. Taking the observed size-frequency relation of Sgr A* into account, these time lags suggest a moderately relativistic (lower estimates: 0.5c for two-sided, 0.77c for one-sided) collimated outflow.